FIG. 1 illustrates a cross-sectional view of a tooth, with several parts of the tooth identified. Many dental or medical procedures, such as cleaning non-vital nerves and blood vessels from a root canal, require that a dental/medical practitioner know the location of the opening of an apex of a patient's tooth. The apex is the tip of the root, and it has an opening or passage where nerve, blood supply, and other tissue leave the jawbone and enter the tooth's root canal. The opening is commonly referred to as the apical foramen. These dental/medical procedures typically involve inserting and using or operating a dental instrument, such as a file or other tool, to remove tissue from the canal. It is important that the position of the tip of the dental instrument be known relative to the apical foramen to minimize tissue damage. Throughout this specification, a “dental instrument” includes any device inserted into a tooth for direct or indirect treatment, including but not limited to reamers, files, and like instruments used to clean out tissue contained in a tooth's root canal, to fix anchors into the tooth, to insert syringe needles and other devices into the tooth, and the like. The dental instrument includes an electrically conductive portion along at least a part of its length.
For example, if the dental/medical practitioner does not clear a root canal by inserting the file or other tool all the way to the apical foramen, nerve, vascular and other tissue can remain in the tooth's root canal. This remaining tissue can become infected and create problems for the patient. If the dental/medical practitioner penetrates the apical foramen, healthy nerve, vascular, and other tissue can be damaged. Such damage can cause unnecessary pain for the patient. Consequently, a number of methods and devices have been developed to help the dental/medical practitioner determine the location of the apical foramen of a specific tooth.
One such method and device includes taking numerous radiographs, such as x-rays with an x-ray machine, of a patient's tooth while the dental/medical practitioner moves a dental tool in the root canal. Unfortunately, this method subjects the patient to multiple exposures of radiation as the dental/medical practitioner moves the dental tool toward the apical foramen. This method can also be very time consuming because the dental/medical practitioner does not move the dental tool while the patient's tooth is radiographed and the radiographs developed. This method can also fail to show the location of the apical foramen relative to a dental tool if the tooth cannot be isolated on a radiograph.
Another such method and device includes electronically detecting the apical foramen's location by measuring changes in impedance (resistance and capacitance) between an electrode in a patient's tooth (often the dental instrument) and an electrode attached to the patient's lip. Typically, a stimulus voltage applied across these electrodes includes two or more signals. One signal has a high frequency while the other signal has a low frequency. Since the capacitive portion of an impedance is a function of signal frequency, impedances at the two frequencies are compared to estimate the capacitive portion. By monitoring changes in the impedance associated with each signal as the dental/medical practitioner moves the tool in the root canal, the dental/medical practitioner can be provided an approximate location of the tooth's apical foramen relative to the tool tip.
With this method, the practitioner must stop the operation of the handpiece to eliminate the electrical noise, maintain the position of the dental instrument in the root canal, attach a lead of an electronic detector to the dental instrument, read the proximity of the dental instrument's tip to the apical foramen, disconnect the lead, and resume operation until another proximity indication is desired, when the process is repeated. Dental/medical practitioners would like to know the proximity of the tip to the apical foramen in real time as they move the tip of the dental instrument down the root canal, particularly when driving the dental instrument with an electrically operated handpiece. Attempts have been made to eliminate the several steps of individually connecting and disconnecting the lead to the dental instrument by externally mounting the electronic detector lead on the handpiece, and coupling the lead by a brush to the dental instrument. Such systems require a custom or retrofitted handpiece, and still require an extra wire. In addition, noise immunity of these systems during handpiece operation has not been established.
Coupling proximity indication circuitry to existing electrically conductive handpiece pathways has encountered problems with electrical noise. Operation of the handpiece creates electrical noise, such as make and break of conductive gears, gears turning, PWM motor drivers, a metal motor, and other equipment. Direct current, which may be more noise immune, should not be used because of possible adverse biological consequences. Existing techniques measure the real component of a proximity-detecting signal (also referred to as “stimulation signal”) passing through a tooth by measuring the peak components of this signal. However, noise from the dental handpiece adds to the peak values and distorts the signal readings, or the signal peaks drift. Existing methods work well only when the dental handpiece is off, and noise from the dental handpiece is absent.